Automatic balancing device

ABSTRACT

The invention provides an automatic balancing device for counterbalancing an out-of-balance mass present in a rotating body. The automatic balancing device ( 50 ) comprises a chamber ( 56 ) having an outer wall ( 54 ) and an axis ( 18 ), first and second counterbalancing masses ( 80   a   , 80   b ) constrained to move freely in a circular path about the axis ( 18 ) and within the chamber ( 56 ), and a viscous fluid ( 60 ) provided in the chamber ( 56 ) so as to provide viscous coupling between the outer wall ( 54 ) of the chamber ( 56 ) and each of the counterbalancing masses ( 80   a   , 80   b ). The counterbalancing masses ( 80   a   , 80   b ) are adapted and/or arranged such that, in use, the first counterbalancing mass ( 80   a ) leads the second counterbalancing mass ( 80   b ). Constraining means ( 90   a   , 92   a   , 94   a   , 94   b ) are provided so as to prevent the first counterbalancing mass ( 80   a ) from leading the second counterbalancing mass ( 80   b ) by more than substantially 180° when the device is in use. This arrangement minimises the excursion of the rotating body at the critical speed. Such an arrangement has application particularly in the field of washing machines in which the position and size of the out-of-balance mass is unpredictable and there is a need to increase the speed of rotation of the body from a relatively low speed to a relatively high speed.

The invention relates to an automatic balancing device for counterbalancing an out-of-balance mass present in a rotating body. Particularly, but not exclusively, the invention relates to an automatic balancing device for use in washing machines and for counterbalancing out-of-balance masses present in the washing machine during washing and spinning cycles.

Automatic balancing devices for counterbalancing out-of-balance masses are known in many different applications. However, the most complex out-of-balance situations occur when both the position and size of the out-of-balance mass is unpredictable and the speed of rotation is variable, as in the case of a washing machine. Many different automatic balancing devices have been proposed and used in washing machines and many of these are effective at counterbalancing out-of-balance masses at speeds above the critical speed (ie. the speed of resonance of the system). Examples of this type of automatic balancing device are shown in GB1,035,033; GB1,092,188; WO 93/23687; WO 95/32372; U.S. Pat. No. 5,813,253; U.S. Pat. No. 5,862,553; DE1 912 481. All of the devices shown in these documents make use of the phenomenon by means of which, at speeds of rotation above the critical speed, freely rotatable counterbalancing masses automatically take up positions in which the out-of-balance mass is counterbalanced. However, it is recognised in some of the aforementioned documents that, at speeds below the critical speed, freely rotating counterbalancing masses act so as to exacerbate the excursion of the rotating body due to the presence of the out-of-balance mass. In these cases, the counterbalancing masses are proposed to be locked in a fixed position with respect to the chamber in which they are located when the body is rotating at a speed below the critical speed. In the case of U.S. Pat. No. 5,813,253, a roller locates in a recess in order to prevent the balancing masses from moving along the annular path in which they run. The roller is released from the recess when the body exceeds the critical speed. In GB1,092,188, the counterbalancing masses are pivotably mounted about an axle with locking members provided to lock the masses in a fixed position with respect to the chamber in which they are housed when the speed of rotation is below critical. The locking means release when the speed is above critical. It is also envisaged in this prior art document that the counterbalancing members could be locked together, so that they have a zero net out-of-balance effect until the critical speed has been exceeded.

The known mechanisms by means of which counterbalancing masses can be locked in a position in which they have a zero net out-of-balance effect are generally difficult and expensive to manufacture. They are susceptible to damage in view of the movement of the counterbalancing masses, which can sometimes be quite violent. However, in arrangements which do not lock the counterbalancing masses, acceleration of the drum of the washing machine from a below-critical speed to an above-critical speed can cause extreme excursion, especially at the critical speed.

It is an object of the invention to provide an automatic balancing device for counterbalancing an out-of-balance mass present in a rotating body in which, during acceleration through the critical speed, the amount of excursion of the rotating body is minimised. It is a further object of the present invention to provide an automatic balancing device which is economic to manufacture and less susceptible to damage than known arrangements. It is a further object of the present invention to provide a method of operating a washing machine in which rotation of the drum can be accelerated through the critical speed with the minimum of excursion.

The invention provides an automatic balancing device for counterbalancing an out-of-balance mass present in a rotating body, the automatic balancing device comprising a chamber having an outer wall and an axis, first and second counterbalancing masses constrained to move freely in a circular path about the axis and within the chamber, and a viscous fluid provided in the chamber so as to provide viscous coupling between the outer wall of the chamber and each of the counterbalancing masses when the device is in use, characterised in that the counterbalancing masses are adapted and/or arranged such that, in use, the first counterbalancing mass leads the second counterbalancing mass, and constraining means are provided so as to prevent the first counterbalancing mass from leading the second counterbalancing mass by more than substantially 180° when the device is in use.

Preferably, the first counterbalancing mass has a moment of inertia which is lower than that of the second counterbalancing mass. More preferably, the viscous coupling between the first counterbalancing mass and the outer wall of the chamber is higher than the viscous coupling between the second counterbalancing mass and the outer wall of the chamber.

The claimed arrangement has been found to be effective in reducing the amount of excursion of the rotating body in comparison to an arrangement utilising no counterbalancing means, at least when used in a washing machine. Indeed, the claimed arrangement is capable of reducing the maximum excursion of the rotating drum of a washing machine to a level significantly below that currently achieved by commercially available washing machines which utilise a known balancing device. The reduction in excursion is particularly important as the rotating body passes through the critical speed as the excursion is largest at this speed. Reducing the excursion, particularly at the critical speed, allows washing machines and other similar devices to be manufactured with larger drums because less provision for excursion needs to be made. Also, less ballast needs to be provided.

The claimed arrangement is also very simple to manufacture and reliable in operation. In place of the known locking arrangements described in the prior art discussed above, the constraining means can take a very simple form which results in higher reliability and lower failure rates than more complex arrangements. The manufacturing cost of the claimed arrangement is also lower than the cost of the known prior art arrangements.

The claimed arrangement is believed to operate in the following manner. When the rotating body is rotated at a speed below critical but sufficient to cause the counterbalancing masses to rotate within the chamber, the counterbalancing masses spread apart because the first counterbalancing mass leads the second counterbalancing mass. As the speed of the device increases, the spread of the masses will increase until, when the spread is at or near 180°, they are prevented from spreading apart any further by the constraining means. In this position, ie. when the masses are spread apart by approximately 180°, they contribute little or nothing to the out-of-balance mass present in the rotating body and the excursion of the rotating body is not exacerbated. However, at the same time, the excursion of the rotating body causes the masses to be drawn towards one another. Because the masses are free to move with respect to the chamber (within the constraints applied by the constraining means), they move towards one another thereby effecting a partial balancing of the out-of-balance mass in the rotating body. This movement of the masses affects the phase and size of the excursion of the rotating body which then causes further movement of the masses in response. Hence the masses are in constant movement with respect to the chamber, continually moving into a position in which the out-of-balance mass is partially balanced. This results in a reduction of the excursion of the rotating body.

It is also believed that, as the speed of rotation of the rotating body approaches resonance (the critical speed), the counterbalancing masses tend towards a position in which the out-of-balance mass is fully balanced. Thus the amount by which the excursion of the rotating body is reduced increases as the rotating body approaches resonance.

The invention also provides a method of operating a washing machine having a drum and incorporating the automatic balancing device described above, the method comprising the steps of:

-   -   (a) rotating the drum and the automatic balancing device at a         speed below the critical speed of the washing machine whilst         allowing each of the counterbalancing masses to rotate freely         about the axis;     -   (b) causing the first counterbalancing mass to lead the second         and any further counterbalancing mass;     -   (c) preventing the first counterbalancing mass from leading the         second and any further counterbalancing mass by more than         substantially 180°; and     -   (d) increasing the speed of rotation of the drum to a speed         above the critical speed of the washing machine.

Preferably, the method comprises the further steps of:

-   -   (e) detecting variations in the amplitude of excursion of the         drum due to the presence of an out-of-balance load therein and         the counterbalancing masses;     -   (f) detecting a minimum amplitude of excursion; and     -   (g) commencing the increase in the speed of rotation of the drum         to a speed above the critical speed of the washing machine at a         time when the amplitude of excursion is at or close to a         minimum.

Preferably, the speed of rotation of the drum is increased from a speed below the critical speed of the washing machine to a speed above the critical speed of the washing machine at a rate of between 5 rpm/s and 50 rpm/s, more preferably between 5 rpm/s and 15 rpm/s.

It is believed that the rate of acceleration of the drum can have an effect on the amount by which the excursion of the rotating body is reduced. Slower rates of acceleration have been shown to improve the effect. It is believed that this is due to the fact that slower rates of acceleration allow the counterbalancing masses time to adopt new, advantageous positions which will minimize the excursion experienced as the rotating body passes through resonance.

Further preferred and advantageous features of the invention are set out in the subsidiary claims.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic sectional side view of a washing machine incorporating an automatic balancing device according to a first embodiment of the invention;

FIG. 2 is a sectional side view through the automatic balancing device shown in FIG. 1;

FIG. 3 a is a plan view of one of the counterbalancing masses forming part of the automatic balancing device shown in FIG. 2;

FIG. 3 b is a perspective view of the counterbalancing mass of FIG. 3 a from the opposite side;

FIG. 4 a illustrates the interaction of two counterbalancing masses of the type shown in FIGS. 3 a and 3 b in use in the automatic balancing device of FIG. 2 in a first position;

FIG. 4 b illustrates the interaction of the same two counterbalancing masses in a second position;

FIG. 5 is a schematic front view of an automatic balancing device according to a second embodiment of the invention;

FIG. 6 is a perspective view of a collar forming part of the device of FIG. 5;

FIG. 7 illustrates the position and operation of the collar of FIG. 6 with respect to two counterbalancing masses forming part of the device shown in FIG. 5;

FIG. 8 a illustrates the positioning of the masses of FIG. 5 inside a chamber;

FIG. 8 b shows the surface of one of the masses of FIG. 5;

FIG. 9 illustrates a third embodiment of the invention; and

FIGS. 10 a, 10 b and 10 c illustrate alternative counterbalancing masses suitable for use in the embodiments illustrated in FIGS. 1 to 9.

FIG. 1 illustrates a typical environment in which an automatic balancing device is useful and desirable. FIG. 1 shows a washing machine 10 having an outer casing 12 and a tub 14 mounted inside the outer casing 12 by way of a system of springs and dampers 15. A perforated drum 16 is mounted inside the tub 14 so as to be rotatable about an axis 18. In this embodiment, the axis 18 extends horizontally although this is not essential. A hinged door 20 is located in the front face of the outer casing 12 in such a manner that, when the door 20 is in a closed position (as illustrated), the tub 14 is sealed in a watertight manner. The door 20 is openable to allow articles of laundry to be placed inside the drum 16 prior to the commencement of a washing cycle to be carried out by the washing machine 10. Flexible seals 22 are also provided between the drum 16 and the door 20 so that moderate movements of the drum 16 with respect to the outer casing 12 can be tolerated.

The drum 16 is mounted in a rotatable manner by way of a shaft 24 which is supported in cantilever fashion in the washing machine 10 and driven by a motor 26. The shaft 24 passes through the tub 14 and into the interior thereof so as to support the drum 16. The drum 16 is fixedly connected to the shaft 24 so as to rotate therewith about the axis 18. It will be understood that the shaft 24 passes through the wall of the tub 14 in such a manner as to cause no rotation of the tub 14. Such mounting arrangements are well known in the art. The washing machine 10 also includes a soap tray 28 for the introduction of detergent, one or more water inlet pipes 30 leading to the tub 14 via the soap tray 28, and a water drain 32 communicating with the lower-portion of the tub 14.

All of the features thus far described in relation to the washing machine 10 are known per se and do not form essential parts of the present invention. Common variants of any or all of these features may therefore be included in a washing machine capable of incorporating or utilising an automatic balancing device according to the invention if desired.

The present invention relates to an automatic balancing device suitable for use in a washing machine of the general type described above. In the embodiment shown, the automatic balancing device 50 is located on the distal end of the shaft 24, inside the drum 16 and adjacent the rear wall 16 a thereof. However, an automatic balancing device 50 of the type described below can also be located on the outside of the drum 16, still adjacent the rear wall 16 a thereof, but on the side facing the tub 14. It is also possible to provide the automatic balancing device at other positions along the shaft 24; for example, between the tub 14 and the outer casing 12, although such an arrangement is not the preferred position.

The automatic balancing device 50 is illustrated in a rest position in FIG. 2. The automatic balancing device 50 has a cup-shaped cover 52 which, together with a circular plate 53 lying next to the rear wall 16 a of the drum 16, forms an outer wall 54 defining a chamber 56. The cover 52 is fixedly attached to the circular plate 53 so that the chamber 56 is liquid-tight for reasons which will be explained below. The means of connection between the cover 52 and the circular plate 53 are not material to the invention. The circular plate 53 can be attached to the rear wall 16 a of the drum 16 if desired. The cover 52 is fixedly attached to the distal end 24 a of the shaft 24 and is rotatable therewith so that, when the shaft 24 is rotated by the motor 26 about the axis 18, the cover 52 is rotated as well. The shaft 24 passes through the chamber 56 from the circular plate 53 to the cover 52.

Two counterbalancing masses 80 a, 80 b are rotatably mounted on the shaft 24. The masses 80 a, 80 b are mounted on the shaft 24 via bearings 58 so that the masses 80 a, 80 b are able to rotate freely about the shaft. Means (not shown) are provided on the shaft 24 for preventing the masses 80 a, 80 b from moving axially along the shaft 24, their positions being maintained with respect to one another and with respect to the cover 52. The chamber 56 contains a pool of oil 60 (or other viscous liquid) which is sufficiently deep to ensure that the counterbalancing masses 80 a, 80 b are partially submerged as shown in FIG. 2. Indeed, it is preferred that there is sufficient oil 60 in the chamber 56 to ensure that, when the automatic balancing device 50 is rotated at a speed sufficient to distribute the oil 60 about the periphery of the chamber 56, the counterbalancing masses 80 a, 80 b are still partially submerged in, or at least in contact with, the oil 60.

The counterbalancing masses 80 a, 80 b are identical to one another. The configuration of one of the masses 80 a is shown in detail in FIGS. 3 a and 3 b. Essentially, the mass 80 a comprises a disc portion 82 a which is relatively slim in depth and circular in plan view. A mass portion 84 a is provided on the disc portion 82 a and is rigidly fixed thereto. Indeed, the mass portion 84 a can be formed integrally with the disc portion 82 a if desired. In the embodiment shown, the mass portion 84 a overlies an area of the disc portion 82 a amounting to approximately one quarter, but the extent of overlie is not important. What is important is that the shape of the mass portion allows the centre of gravity of the mass 80 a to be located a significant distance from the centre of the disc portion 82 a. A central aperture 86 a is located in the geometric centre of the disc portion 82 a and passes through both the disc portion 82 a and the mass portion 84 a. The aperture 86 a is dimensioned so as to receive the bearings 58 by means of which the mass 80 a is mounted on the shaft 24. The function of the mass portion 84 a is to provide the counterbalancing mass 80 a with an eccentric centre of gravity and sufficient mass to allow the mass 80 a to function as a counterbalance to an out-of-balance load present in the drum 16 of the washing machine 10 during its operation.

The mass portion 84 a is delimited by an edge or lip 88 a. The edge or lip 88 a comprises a surface which extends generally perpendicular to the circular faces of the disc portion 82 a. The edge or lip 88 a includes two abutment surfaces 90 a, 92 a whose function will be described below.

The mass 80 a carries a pin 94 a which protrudes beyond the surface of the disc portion 82 a but does not protrude beyond the surface of the mass portion 84 a. The pin 94 a extends beyond the surface of the disc portion 82 a, and perpendicular thereto, by an amount which is greater than the smallest distance a between the counterbalancing masses 80 a, 80 b as shown in FIG. 2. However, the distance protruded by the pin 94 a is not as great as the larger distance A between the two masses 80 a, 80 b, as will be explained below.

The counterbalancing mass 80 b is identical to the counterbalancing mass 80 a described above. Parts of the counterbalancing mass 80 b will now be referred to using the reference numeral assigned to the corresponding part of mass 80 a, but with the letter “a” replaced by the letter “b”.

The arrangement of the counterbalancing masses 80 a, 80 b in the automatic balancing device 50 is shown in FIG. 2. The first counterbalancing mass 80 a is positioned to the left as shown and the second counterbalancing mass 80 b is positioned to the right. As has been mentioned, the distance a between the masses 80 a, 80 b in the area where the mass portions 84 a, 84 b overlap is less than the distance by which the pin 94 b protrudes beyond the disc portion 82 b of the mass 80 b. However, the distance A between the disc portions 82 a, 82 b is greater than the extent of protrusion of the pin 94 b from the disc portion 82 b. Thus the distal end of the pin 92 b is constrained to travel between the abutment surfaces 90 a, 92 a passing across the disc portion 82 a and not across the mass portion 84 a. At each end of the possible travel path of the pin 94 b relative to the mass 80 a, the pin 94 b will contact one of the two abutment surfaces 90 a, 92 a of the other mass 80 a.

The rest position of the two masses 80 a, 80 b is shown in FIG. 4 a. The fact that the masses 80 a, 80 b are identical means that, in the absence of the pin 94 b, the masses 80 a, 80 b would lie alongside one another in alignment. However, because the pin 94 b may not lie alongside the mass portion 84 a of the mass 80 a, it abuts against the abutment surface 90 a of the mass 80 a. Hence the masses 80 a, 80 b lie slightly out of alignment in the rest position as shown in FIG. 4 a. It will be appreciated that, in the arrangement shown in FIG. 2, the pin 92 a of the mass 80 a is in fact redundant.

In operation, the automatic balancing device 50 operates as follows. Rotation of the drum 16 is effected by rotation of the shaft 24. The automatic balancing device 50 rotates with the shaft 24 and the drum 16 so that the whole of the outer wall 54 of the chamber 56 rotates at a relatively high speed. By this we mean that the speed of rotation of the drum 16, and thus the chamber 56, is sufficient to create centrifugal forces which will overcome gravitational forces and so maintain the load contained within the drum pressed against the wall thereof but is below the critical speed of the washing machine. Hence the counterbalancing masses 80 a, 80 b are not in a position to perform an automatic counterbalancing function. However, the viscous coupling provided by the oil 60 between the outer wall 54 of the chamber 56 and the counterbalancing masses 80 a, 80 b will cause the counterbalancing masses 80 a, 80 b to rotate about the shaft 24. Due to the dynamics of the system, the masses 80 a, 80 b will rotate about the shaft 24 at a rotational speed which is lower than that of the shaft 24 and the drum 16. Because of the shaping of the masses 80 a, 80 b, and particularly the fact that a significant proportion of the surface of the mass 80 b facing the circular plate 53 is spaced further from the circular plate 53 than the whole of the surface of the mass 80 a facing the cover 52, the viscous coupling between the outer wall 54 and the mass 80 a will be higher than the viscous coupling between the outer wall 54 and the mass 80 b. Hence, the mass 80 a will rotate at a speed closer to that of the drum 16 and the chamber 56 than the mass 80 b. The result of this is that the mass 80 a will become the leading mass and that the mass 80 b will trail behind it. Moreover, the mass 80 a will rotate about the shaft 24 at a higher angular velocity than the mass 80 b. Furthermore, the abutment of the pin 94 b against the abutment surface 90 a will eliminate any possibility of the mass 80 a from lagging behind the mass 80 b. In the embodiment shown, the location of the pin 94 b and the abutment surface 90 a will in fact ensure that the mass 80 a always leads the mass 80 b.

As the drum 16 and chamber 56 rotate about the shaft 24 in the direction shown by the arrow B in FIG. 4 b, the trailing mass 80 b will trail further behind the leading mass 80 a. The masses 80 a, 80 b thus spread apart with the mass 80 a leading and the mass 80 b trailing behind it by an increasing amount. However, as the angle by which the mass 80 b trails the mass 80 a approaches 180° (see FIG. 4 b), the pin 94 b abuts against the abutment surface 90 a thus preventing the mass 80 b from lagging behind the mass 80 a by more than 180°. In the embodiment shown, the mass 80 b is constrained to lag the mass 80 b by less than 180°. Even so, the combined effect of the counterbalancing masses 80 a, 80 b on the rotating body in this position is small.

It has been found that, by allowing the counterbalancing masses 80 a, 80 b to rotate freely within the constraints described above, the maximum amount of excursion of the drum 16 can be kept to a minimum as the drum 16 is accelerated from the speed described above, through the critical speed to a spin speed high enough to extract water from the load carried in the drum 16. Spin speeds are commonly 1400 to 1600 rpm at present. It is advantageous if the rate of acceleration of the drum 16 is kept to a moderate rate: a rate of increase of speed of between 5 and 50 rpm per second is generally regarded as favorable, with a rate of increase of 5 to 15 rpm/s being more favorable still. Hence, in operation, the speed of the drum 16 is increased at a rate of between 5 and 50 rpm/s (preferably between 5 and 15 rpm/s) from the relatively high speed mentioned above to a suitable spin speed (typically 1400 to 1600 rpm) without constraining or otherwise locking the counterbalancing masses 80 a, 80 b to one another or to the outer wall 54 of the chamber 56. In this way, the excursion of the drum 16 at the critical speed is reduced in comparison to other systems. This allows the dimensions of the drum 16 to be maximized for any given size of washing machine and/or the risk of damage occurring when large loads are spun at high speeds to be minimized. Above the critical speed, the masses 80 a, 80 b automatically adopt positions which will counterbalance any out-of-balance present in the drum 16, as is well known.

Optionally, a sensing device 27 (shown in dotted lines in FIG. 1) may be connected to the motor 26. The sensing device 27 detects the current drawn by the motor 26 and/or the running speed of the motor 26. As the masses 80 a, 80 b adopt different relative positions, and thus perform a greater or lesser counterbalancing effect over time, either a minimum current drawn by the motor 26 or a maximum speed of rotation of the motor 26 will be indicative of a position of greatest counterbalancing effect. At these positions, the excursion of the drum 16 will be at a minimum for a given speed. It is believed to be beneficial to detect when these periods of minimum excursion occur and to initiate the acceleration of the drum 16 to the required spin speed at a point in time when the excursion is at or close to a minimum. Experimental results have shown that initiating the acceleration at such a time is advantageous in many cases.

In a method of operating the washing machine of FIG. 1, when it is desired to rotate the drum at speeds high enough to extract wash liquor or rinse water by spinning, the drum is first rotated at a speed which is sufficient to stick the washload to the walls of the drum but below the critical speed. The masses 80 a, 80 b are allowed to rotate freely about the shaft 24, although the engagement of the pin 94 b against abutment surfaces 90 a, 92 a prevents the mass 80 b from leading the mass 80 a at all or from lagging the mass 80 a by more than 180°. The difference in the viscous coupling between the outer wall 54 of the chamber 56 and each of the masses 80 a, 80 b causes the masses initially to spread apart. Thereafter, the masses 80 a, 80 b will redistribute themselves under the influence of various forces and will, periodically, take up positions which cause maxima and minima of excursion of the drum 16 and the shaft 24. If the motor 26 has coupled to it a sensing device 27 as described above, the speed of and/or current drawn by the motor 26 is monitored and a minimum value of either characteristic is sensed. The drum 16 is then accelerated to the desired spin speed. This will inevitably require the speed of rotation of the drum to pass through the critical speed, at which the excursion of the drum 16 is greatest. The rate of acceleration is between 5 and 15 rpm/s but could be as high as 50 rpm/s. By allowing the masses 80 a, 80 b freedom to rotate about the shaft 24 during the acceleration step, the maximum excursion is kept as low as possible. Also, by initiating the acceleration of the drum 16 from the lower speed at a point when the excursion is at or close to a minimum (as sensed by the sensor 27), the excursion experienced by the drum 16 at the critical speed is minimized. Above the critical speed, the masses 80 a, 80 b position themselves, as is well known, so as to counterbalance the out-of-balance load within the drum 16 and so the excursion of the drum 16 at speeds above critical is greatly reduced and, in some cases, eliminated.

Keeping the maximum excursion of the drum 16 to a minimum is beneficial because a lower provision for excursion then needs be built into the machine and thus the size of the drum 16 can be increased in comparison to other machines. Also, the risk of damage occurring due to excessive excursion of the drum 16 is reduced.

A second embodiment of the invention is illustrated in FIGS. 5, 6 and 7. FIG. 5 is a cutaway front view of an automatic balancing device 150 having an outer wall 154 delimiting a cylindrical chamber 156. A shaft 124, which is rotatable about an axis 118, passes through the chamber 156. Two counterbalancing masses 180 a, 180 b are mounted on the shaft 124 via bearings 158 so as to be freely rotatable about the shaft 124. The counterbalancing masses 180 a, 180 b are shaped so as to increase in breadth with distance from the shaft 124. As with the masses 80 a, 80 b illustrated in FIGS. 2, 3 and 4, the effect of this is to space the centre of gravity of each counterbalancing mass 180 a, 180 b away from the axis 118.

Located between the counterbalancing masses 180 a, 180 b is a collar 190 which is shaped and dimensioned to be supported on and freely rotatable about the shaft 124.

Projecting outwardly from a first annular face 192 a of the collar 190 are two diametrically opposed pins 194 a. A single pin 194 b projects outwardly from a second annular face 192 b of the collar 190, the pin 194 b being aligned with one of the pins 194 a. The pins 194 a, 194 b project sufficiently far from the respective annular surfaces 192 a,192 b to engage with the counterbalancing masses 180 a, 180 b as they rotate about the shaft 124.

The dimensions of the collar 190 and the positioning of the pins 194 a, 194 b thereon are such that the pins 194 a abut against the edges of the counterbalancing mass 180 a with little or no play as shown in FIG. 5. Thus the collar is held in a substantially fixed position with respect to the counterbalancing mass 180 a. However, since the counterbalancing mass 180 b is acted upon by only the single pin 194 b, it is able to move relative to the counterbalancing mass 180 a between a first position in which the masses 180 a, 180 b are aligned and a second position in which the masses 180 a, 180 b are diametrically opposed. The positioning of the pin 194 b is such that the mass 180 a is not allowed to lag behind the mass 180 b. As in the previous embodiment, the viscous coupling between the outer wall 154 and the mass 180 a is higher than that between the outer wall 154 and the mass 180 b. This can be achieved in any suitable way.

One way to vary the viscous coupling between the outer wall 154 and the respective masses 180 a, 180 b is illustrated in FIG. 8 a. Here, the mass 180 a is arranged so as to be closer to the outer wall 154 than the mass 180 b. An alternative way of ensuring that the viscous coupling between the outer wall 154 and the mass 180 a is higher than that between the outer wall 154 and the mass 180 b is to provide the surface of the mass 180 a with an uneven or textured finish, as illustrated in FIG. 8 b. This non-planar finish need only be applied to the part of the surface of the mass 180 a facing the outer wall, if desired, although more of the surface can be finished in this way. Further alternative means for ensuring that the viscous coupling between the outer wall 154 and the mass 180 a is higher than that between the outer wall 154 and the mass 180 b will be apparent to a skilled reader.

In the embodiments described above, only two counterbalancing masses have been provided. It is possible to provide more than two masses and an arrangement showing the use of three masses is shown in FIG. 9. Each mass 280 a, 280 b, 280 c is mounted in a chamber 256 on a shaft 224 so as to be freely rotatable thereabout. Collars 290 a, 290 b are mounted between each pair of masses 280 a, 280 b and 280 b, 280 c as shown. The collars 290 a, 290 b are similar to the collar 190 shown in FIG. 6 and described above and operate in the same way. However, it is to be noted that the side of each collar 290 a, 290 b from which two diametrically opposed pins project is arranged so as to face the central mass 280 b. This ensures that the other masses 280 a, 280 c are prevented from leading the central mass 280 b or from lagging the central mass 280 b by more than 180°. In order to ensure that the viscous coupling between the outer wall 254 of the chamber 256 and the central mass 280 b is higher than the viscous coupling between the outer wall 254 of the chamber 256 and the outer masses 280 a, 280 c, fins 281 are provided on the outer edge of the mass 280 b. The distal edges of the fins 281 lie very close to the outer wall 254 of the chamber 256 and provide high viscous coupling for the central mass 280 b.

Each of the embodiments described above makes use of a plurality of counterbalancing masses which are identical to one another. By this we mean that the masses are identical in shape and are made from material of the same density so that the centres of gravity of each mass lie at the same point. Different arrangements for ensuring that the first (leading) mass has a different viscous coupling with the wall of the chamber from that of the or each other mass have been described above. However, it is possible to arrange for the first mass always to lead the or each other mass in different ways. One such way is to provide the first mass with a lower moment of inertia than the or each other mass. This can be achieved by ensuring that the centre of gravity of the first counterbalancing mass lies closer to the axis of the chamber than the centre of gravity of the or each other mass. One way to achieve this is to arrange for the mass of the first counterbalancing mass to be lower than that of the or each other counterbalancing mass. This is possible by manufacturing the first counterbalancing mass (or a part thereof) from a material having a lower density than that of the other counterbalancing mass or masses; by providing the mass portion 382 a of the first counterbalancing mass 380 a with one or more hollow cavities 383 a (see FIG. 10 a); or by providing the first counterbalancing mass 480 a with a number of mass-reducing through-holes 481 a (see FIG. 10 b). Alternatively, the mass portion 582 a of the first counterbalancing mass 580 a can be shaped and configured so that, with respect to the or each other counterbalancing mass, the centre of gravity lies closer to the axis 518 than that of any other counterbalancing mass used in the same system (see FIG. 10 c in which the shape of the mass portion 582 b of another counterbalancing mass is shown in dotted outline). In this arrangement, it is envisaged that the mass of each counterbalancing mass is the same as that of the other counterbalancing masses.

Thus it can be seen that the first counterbalancing mass can be arranged to lead the or each remaining counterbalancing mass by providing it with either a higher viscous coupling with the chamber wall or a lower moment of inertia than that of the or each remaining counterbalancing mass. Other means for arranging for the first counterbalancing mass to lead the or each remaining counterbalancing mass will be apparent to a skilled reader.

The skilled reader will also appreciate that it is possible to rely solely on the means for arranging for the first counterbalancing mass to lead the or each remaining counterbalancing mass and to dispense with any further constraining means which will physically prevent the first counterbalancing mass from lagging the or any other counterbalancing mass. Referring to the embodiment shown in FIGS. 1 to 4, the abutment of the pin 92 a against the abutment surface 90 a prevents the first counterbalancing mass 80 a from lagging the second counterbalancing mass 80 b. However, it is possible to rely on the higher viscous coupling between the first counterbalancing mass 80 a and the chamber wall 54 to ensure that the first counterbalancing mass 80 a leads the second counterbalancing mass 80 b and so the abutment surface 90 a can be dispensed with. A similar change could be made to the embodiments illustrated in FIGS. 5 to 10.

The scope of the invention is not limited to the embodiments described above. It will be appreciated that the shape of the counterbalancing masses can be varied almost indefinitely as long as the essential counterbalancing function is achieved. Indeed, the counterbalancing masses provided in a single automatic balancing device as described above may be different in shape from one another and do not have to be identical. Different numbers of masses may be provided and alternative means for constraining the relative position of the masses may be provided. A further alternative arrangement contemplated within the scope of the invention is the use of the rear wall of the drum to form part of the outer wall of the chamber of the automatic balancing device. Other variations will be apparent to a skilled reader. 

1. An automatic balancing device for counterbalancing an out-of-balance mass present in a rotating body, comprising a chamber having an outer wall and an axis, first and second counterbalancing masses constrained to move freely in a circular path about the axis and within the chamber such that, in use, the first counterbalancing mass leads the second counterbalancing mass, a viscous fluid provided in the chamber so as to provide viscous coupling between the outer wall of the chamber and each of the counterbalancing masses when the device is in use and first constraints arranged so as to prevent the first counterbalancing mass from leading the second counterbalancing mass by more than substantially 180° when the device is in use.
 2. An automatic balancing device as claimed in claim 1, wherein the first constraints are adapted to ensure that the angle by which the second counterbalancing mass lags the first counterbalancing mass is always less than substantially 180°.
 3. An automatic balancing device as claimed in claim 1 or 2, further comprising second constraints to prevent the first counterbalancing mass from lagging behind the second counterbalancing mass.
 4. An automatic balancing device as claimed in claim 3, wherein the second constraints are adapted to ensure that the first counterbalancing mass always leads the second counterbalancing mass.
 5. An automatic balancing device as claimed in claim 1 or 2, wherein the first counterbalancing mass has a moment of inertia which is lower than that of the second counterbalancing mass.
 6. An automatic balancing device as claimed in claim 5, wherein the centre of mass of the first counterbalancing mass lies closer to the axis than the centre of mass of the second counterbalancing mass.
 7. An automatic balancing device as claimed in claim 1 or 2, wherein the viscous coupling between the first counterbalancing mass and the outer wall of the chamber is higher than the viscous coupling between the second counterbalancing mass and the outer wall of the chamber.
 8. An automatic balancing device as claimed in claim 7, wherein the configuration of the first and second counterbalancing masses is adapted to cause the viscous coupling between the first counterbalancing mass and the outer wall of the chamber to be higher than the viscous coupling between the second counterbalancing mass and the outer wall of the chamber.
 9. An automatic balancing device as claimed in claim 8, wherein, in use, the viscous fluid is in contact with a larger surface area of the first counterbalancing mass than the second counterbalancing mass.
 10. An automatic balancing device as claimed in claim 8, wherein the surface of the first counterbalancing mass has a shaped or textured portion which faces the outer wall of the chamber.
 11. An automatic balancing device as claimed in claim 8, wherein the first counterbalancing mass is located closer to the outer wall of the chamber than the second counterbalancing mass.
 12. An automatic balancing device as claimed in claim 1 or 2, wherein the first and second counterbalancing masses are supported in the chamber on an axle extending concentrically with the axis.
 13. An automatic balancing device as claimed in claim 1 or 2, wherein the first and second counterbalancing masses are spaced along the axis.
 14. An automatic balancing device as claimed in claim 1 or 2, wherein the first constraints are provided on the first and second counterbalancing masses.
 15. An automatic balancing device as claimed in claim 14, wherein the first constraints comprise at least one projection extending from the surface of one of the counterbalancing masses towards the other counterbalancing mass so as to engage with at least one abutment surface of the other counterbalancing mass.
 16. An automatic balancing device as claimed in claim 15, wherein the other counterbalancing mass has a first abutment surface located and arranged such that, when a projection engages therewith, the first counterbalancing mass is prevented from leading the second counterbalancing mass by more than substantially 180°.
 17. An automatic balancing device as claimed in claim 16, wherein the other counterbalancing mass has a second abutment surface located and arranged such that, when a projection engages therewith, the first counterbalancing mass is prevented from lagging behind the second counterbalancing mass.
 18. An automatic balancing device as claimed in claim 16, wherein the abutment surfaces are located on the first counterbalancing mass and the projection is located on the second counterbalancing mass.
 19. An automatic balancing device as claimed in claim 1 or 2, wherein the first constraints are provided on a collar located between the first and second counterbalancing masses.
 20. An automatic balancing device as claimed in claim 19, wherein the first constraints comprise first and second projections extending outwardly from the collar so as to engage with shapings of the first and second counterbalancing masses and thereby prevent the first counterbalancing mass from leading the second counterbalancing mass by more than substantially 180°.
 21. An automatic balancing device as claimed in claim 3, wherein the second constraints comprise first and second projections extending outwardly from the collar so as to engage with shapings of the first and second counterbalancing masses and thereby prevent the first counterbalancing mass from lagging behind the second counterbalancing mass.
 22. An automatic balancing device as claimed in claim 1 or 2, further comprising at least one further counterbalancing mass is provided in the chamber, the arrangement being such that, in use, the first counterbalancing mass leads the further counterbalancing mass and the first constraints are adapted to prevent the first counterbalancing mass from leading the or any further counterbalancing mass by more than substantially 180° when the device is in use.
 23. An automatic balancing device as claimed in claim 22, wherein the viscous coupling between the further counterbalancing mass and the outer wall of the chamber is less than the viscous coupling between the first counterbalancing mass and the outer wall of the chamber.
 24. An automatic balancing device as claimed in claim 22, wherein the moment of inertia of the further counterbalancing mass is higher than the moment of inertia of the first counterbalancing mass.
 25. An automatic balancing device as claimed claim 22, wherein the first constraints are adapted to prevent the first counterbalancing mass from lagging behind the further counterbalancing mass.
 26. An automatic balancing device as claimed in claim 22, further comprising one counterbalancing mass provided in the chamber.
 27. An automatic balancing device as claimed in claim 1 or 2, wherein the viscous fluid is an oil.
 28. (canceled)
 29. A washing machine comprising a drum rotatable about an axis and an automatic balancing device as claimed in claim 1 or 2, the chamber and the drum being arranged so that their axes are collinear.
 30. A method of operating a washing machine comprising a drum rotatable about an axis and an automatic balancing device comprising a chamber having an outer wall and an axis, first and second counterbalancing masses constrained to move freely in a circular path about the axis and within the chamber such that, in use, the first counterbalancing mass leads the second counterbalancing mass, a viscous fluid provided in the chamber so as to provide viscous coupling between the outer wall of the chamber and each of the counterbalancing masses when the device is in use and first constraints arranged so as to prevent the first counterbalancing mass from leading the second counterbalancing mass by more than substantially 180° when the device is in use, the chamber and the drum being arranged so that their axes are collinear, comprising the steps of: (a) rotating the drum and the automatic balancing device at a speed below a critical speed of the washing machine while allowing the counterbalancing masses to rotate freely about the axis; (b) causing the first counterbalancing mass to lead the second counterbalancing mass; (c) preventing the first counterbalancing mass from leading the second counterbalancing mass by more than substantially 180°; and (d) increasing the speed of rotation of the drum to a speed above the critical speed of the washing machine.
 31. A method as claimed in claim 30, wherein the first counterbalancing mass is caused to lead the second counterbalancing mass by causing the first counterbalancing mass to rotate at a higher speed than the second counterbalancing mass.
 32. A method as claimed in claim 30, comprising the further steps of: (e) detecting variations in the amplitude of excursion of the drum due to the presence of an out-of-balance load therein and the counterbalancing masses; (f) detecting a minimum amplitude of excursion; and (g) increasing the speed of rotation of the drum to a speed above the critical speed of the washing machine, the increase in speed being commenced at a time when the amplitude of excursion is at or close to a minimum.
 33. A method as claimed in claim 32, further comprising using a motor forming part of the washing machine to cause the drum to rotate and detecting the amplitude of excursion by measuring the speed of rotation of the motor.
 34. A method as claimed in claim 32, further comprising using a motor forming part of the washing machine to cause the drum to rotate and detecting the amplitude of excursion by measuring the current drawn by the motor.
 35. A method as claimed in any one of claims 30 to 34, wherein the speed of rotation of the drum is increased from a speed below the critical speed of the washing machine to a speed above the critical speed of the washing machine at a rate of between 5 rpm/s and 50 rpm/s.
 36. A method as claimed in claim 35, wherein the speed of rotation of the drum is increased from a speed below the critical speed of the washing machine to a speed above the critical speed of the washing machine at a rate of between 5 rpm/s and 15 rpm/s.
 37. (canceled) 